Yellow, say, on a climbing herb (7)
I believe the answer is:
oregano
'herb' is the definition.
(oregano is a kind of herb)
'yellow say on a climbing' is the wordplay.
'yellow' becomes 'or' (heraldic term).
'say' becomes 'eg' (**).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'on'+'a'='ona'
'ona' back-to-front is 'ano'.
'or'+'eg'+'ano'='OREGANO'
(Other definitions for oregano that I've seen before include "Pizza herb" , "Herb - a no-goer (anag)" , "that's aromatic" , "Marjoram-like herb" , "Wild marjoram" .)